This invention relates to a magnetic powder mainly formed of a magnetic particle made of a soft magnetic metal and having a flat shape.
A composite magnet is used to prevent an electromagnetic interference caused by an electronic device. The composite magnet is typically made of magnetic powder particles and a binder binding the magnetic powder particles. The binder is made of a polymer. Each of the magnetic powder particles is made of a soft magnetic metal. Moreover, each of the magnetic powder particles has a flat shape so as to have an improved magnetic permeability due to the flat shape. Recently, as the electronic device is required to work at higher frequency, the composite magnet is required to correspond to wider frequency range.
For example, a technique related to the composite magnet is disclosed in each of JP-A H10 (1998)-92621 (Patent Document 1) and JP-A 2002-050511 (Patent Document 2), contents of which are incorporated herein by reference.
Patent Document 1 discloses a composite magnet including flat magnetic particles and a binder. Each of the flat magnetic particles has a surface coated with fine particles. According to Patent Document 1, the thus-formed composite magnet has not only a higher electrical resistance but also a higher magnetic permeability at high frequency.
Patent Document 2 discloses a soft magnetic material made of magnetic particles and glass fine particles. Each of the magnetic particles is made of a soft magnetic metal. The glass fine particles are pressed against and attached to the surfaces of the magnetic particles by pressure and friction force. According to Patent Document 2, the thus-formed soft magnetic material has sufficient insulating properties.
When the flat magnetic powder particles of Patent Document 1 are coated with the fine particles, some of the flat magnetic powder particles may be stacked on one another. Each of the thus-stacked flat magnetic powder particles has an exposed surface and an unexposed surface. Only the exposed surface of the magnetic powder particle is coated with the fine particles. In order to completely coat the unexposed surfaces, it is necessary to stir the flat magnetic powder particles repeatedly after the coating process. It takes too much time to coat all of the flat magnetic powder particles. Thus, the technique disclosed in Patent Document 1 has a problem in mass productivity.
Patent Document 2 only discloses a method for attaching the glass fine particles to the magnetic particles by using a surface improving device. Patent Document 2 does not disclose a method for obtaining a flat shaped magnetic particle. In detail, the magnetic particles of Patent Document 2 are pressed so as to be attached with the glass fine particles. The pressed magnetic particles are stirred. The pressing and the stirring are performed repeatedly in the attaching process. Even if the magnetic particle has a flat shape before the attaching process, the flat shape may be deformed during the attaching process. Accordingly, the technique disclosed in Patent Document 2 is not suitable to obtain a flat magnetic powder.